1. Field of the Invention
This invention relates to systems for desulfurization of a flue gas containing sulfur oxides and which may also contain fly ash, and more particularly provides desulfurization systems incorporating a spray dryer absorber and selective recycle of fractions of the spray drying product through the system.
2. Description of the Prior Art
So-called dry desulfurization or dry scrubbing processes are well known and in recent years have achieved increased commercial success relative to so-called wet desulfurization processes. Dry processes are particularly useful in treating flue gas discharged from utility and industrial boilers. These processes typically combine a sprayed liquid and the hot flue gas in a spray dryer absorber and produce a dry powder and gas depleted in sulfur compounds. The sulfur is fixed in the dry powder generally in the form of sulfites and sulfates. The liquid reagent which is sprayed typically is a solution of a soluble strong base or a slurry such as calcium hydroxide.
The economic viability of such systems depends in large part on the stoichiometry achievable with a given reagent. A system which achieves relatively good stoichiometry with a lime reagent is described in U.S. Pat. No. 4,279,873. In that system an aqueous feed including calcium hydroxide is fed to an atomizer and atomized into a flue gas containing fly ash. Conventionally, the powder is separated into two portions within the spray dryer absorber for transport from the spray dryer absorber. A portion of the separated dry powder formed by the atomizer is recycled directly to an aqueous feed preparation system and another portion is transferred to a gas-particulate filter such as a baghouse or an electrostatic precipitator. The powder in each of the portions is similar, there being no discrimination in the transferring process. Some of the powder collected in the gas-particulate filter is subsequently also recycled to the feed preparation system. A part of both the directly recycled powder and the powder removed in the filter is discharged, including an amount of unreacted reagent.
The referenced teaching attributes, at least in part, the achievement of acceptable lime stoichiometric ratios to the recirculation of fine fly ash particles which are especially efficient in acting as carriers for fresh lime because of their smaller size as compared to particles formed by absorption and spray drying. It is noted in the disclosure that the small size has proved to be optimum for performance of the carrier function. A great amount of particles is also stated in the disclosure to have a markedly stabilizing effect in maintaining the fine calcium hydroxide particles in suspension. It also appears that alkalinity in the recycled fly ash is used in the process.
While the referenced disclosure teaches a system which does achieve desulfurization of flue gas at advantageous stoichiometric ratios, further improvements in stoichiometric ratios and reagent consumption are desirable.